Download Solar Furnaces

Solar Furnaces
Today’s Lecture:
Interiors of Stars (Chapter 12, pages 276-295)
• Pre-main sequence stars
• How do stars burn their fuel?
• Post main-sequence evolution
Pre-main-sequence stars
• Relatively slow gravitational contraction, but still no nuclear
reactions.
• Gravitational energy still being released as the gas
compresses.
• When the temperature becomes sufficiently high in the
center (T > 106 K), get nuclear reactions: A STAR IS BORN!
• Star settles onto the main sequence and contraction
ceases. The star’s L and T don’t change much during the
main-sequence lifetime.
• Nuclear reactions replenish energy lost from the surface,
providing stability.
• Star is in mechanical balance: hydrostatic equilibrium.
• Hydrostatic balance does NOT require nuclear reactions.
It’s just pressure balancing gravity.
• But without another energy source the main sequence
would be much shorter (only 30 millions years for our Sun).
Brown Dwarfs: no long-term fusion
• If M < 0.08 Msun, then T is not high enough for
sustained nuclear reactions: get a
“BROWN DWARF” (failed star)
(But fusion does occur for a short time.)
• It is held up by “degeneracy pressure” (quantum
mechanical pressure).
• Until the mid-1990s, no confirmed brown dwarfs had
been found, but now there are over 1000 known.
• They glow faintly at infrared wavelengths.
On the main sequence
• In center of a star, the temperature is very high (in
the Sun, T=1.5 x 107 K). Hydrogen atoms are fully
ionized, so they’re basically just protons.
• Nuclear fusion produces energy.
• Basic reaction is 4 1H1 --> 2He4 + energy
2 of the protons turned into neutrons + positrons
(antielectrons); positrons annihilated with electrons.
• 2He4 is more tightly bound (and therefore less
massive) than 4 1H1, so energy is emitted.
The proton-proton chain (or pp chain)
On the main sequence (cont.)
• 0.7% of the mass of 4 1H1 is converted to energy to
produce 2He4 according to Einstein’s E = mc2.
• The protons (1H1) can only overcome the electric
repulsion (because they’re all positively charged) if
they are moving very fast.
• This means nuclear fusion requires high
temperatures.
(For all you quantum mechanics aficionados, it
actually requires “quantum tunneling” to combine the
protons.)
• Energy released replenishes energy lost from
surface, preventing further contraction.
Sun’s Life on the Main Sequence
• In the Sun, nearly 700 million tons of protons (hydrogen
nuclei) are being converted to helium each second!
• But there is plenty of raw material:
Sun’s core has about 15% of Sun’s mass
Sun is mostly hydrogen: 70% H, 28% He, 2%
heavier stuff
• Sun’s main-sequence life about 10 billion years!
• Note that photons take about 105 years just to leak out.
• All main sequence stars are fusing H to He, but in stars
more massive than 1.5 Msun, the pp chain is not fast
enough. How do we fuse H in this case?
• The CNO cycle
burns hydrogen for
stars > 1.5Msun.
• Carbon acts as a
catalyst. It is
neither created nor
destroyed while
turning four protons
into one helium
atom.
Deaths of stars, 0.08Msun<M<8Msun
• For 10 billions years the Sun lives happily on the main
sequence.
• During this time the Sun is in hydrostatic balance
(gravity pulling in is balanced by pressure pushing out).
• Energy is lost from the surface, but nuclear reactions
provide energy to prevent contraction.
• But eventually a helium core builds up to 0.1Msun, and
there isn’t enough hydrogen left in the core for appreciable
burning.
• Contraction begin in core -> heating the star -> hydrogen
burning becomes stronger in surface layers.
• Star bloats to a huge size: RED GIANT!
RED GIANTS!
Expanding H
envelope
H-burning
shell
He Core
• Contracting He core heats up --> eventually 108 K is reached
3 2He4 --> 6C12 + energy (triple-alpha process)
Also 6C12 + 2He4 --> 8O16 + energy
• Carbon and oxygen core forms over 106 years.
BIGGER RED GIANTS!
• Once again, T is too low
for C/O core fusion, so the
core contracts.
• Off-center burning
expands envelope again,
creating an even larger Red
Giant.
H-burning shell
He-burning shell
CO
• Depending on the mass of
the star, this process can
repeat, creating heavier
elements.
• For stars like our Sun, it becomes unstable and begins
ejecting the outer layers: planetary nebula.
Planetary Nebula
Expanding
shell of gas
• Winds from the star create
a planetary nebula.
• The star is mostly carbon
and oxygen inside, with a
helium layer. Most of the
hydrogen is being expelled.
• An inner core of carbon
and oxygen (0.6-0.9Msun) is
left over, held up by
“degeneracy pressure.”
• This is left over as a
WHITE DWARF star.
CO
White dwarfs
• Roughly the size of the Earth with the mass of the Sun!
• If you try to pack electrons into the same place they must be
at different energy levels (like the energy levels of an atom).
Each electron must be at a higher energy than the one before it.
• All these energetic electrons in one place give rise to a
pressure: ELECTRON DEGENERACY PRESSURE
• This is weird stuff: one teaspoon of white dwarf weighs 3 tons!
If a white dwarf is more massive, it actually has a smaller
radius.
• No nuclear reactions are taking place, the white dwarf just
radiates its heat and continues to cool over time.
• White dwarfs are sometimes used as age indicators in
globular clusters.
Types of White Dwarfs
• The Sun will become a carbon/oxygen white dwarf with a
mass of 0.6Msun.
• Stars up to 8Msun become carbon/oxygen white dwarfs
with masses up to ~1.1Msun.
• Stars below 0.45Msun aren’t massive enough to burn
helium in their core and become helium white dwarfs.
• Stars with masses from 8-10Msun have an extra stage of
burning in their core and make oxygen/neon/magnesium
white dwarfs with masses of ~1.2Msun.
• White dwarfs have a mass limit 1.4Msun (the
Chandrasekhar limit), above which electron degeneracy
pressure can’t hold up the star.